TODAY

A Justin Bieber fan gets hysterical at the concert.

They’d been camping on the streets of New York City for three days -- throngs of teen and tween girls, who had gone without showers (except for the ones that fell from the sky) in the hopes of getting a front row glimpse of their favorite teenage heartthrob.

And when everyone’s imaginary “Boyfriend” Justin Bieber took the stage this morning to play a mini-concert for TODAY, these super-fans did what teen groupies have done for eternity: they screamed, they cried, they quivered. 

So what's behind all the shrieking and sobbing, the shaking and swooning? Is there a physiological explanation for this fan-demonium or is it simply tween and teen girls gone wild?

"It's basically a group response set off by emotions and hormones," says Dr. Sarah Pitts, a specialist in adolescent medicine at Boston Children's Hospital.

TODAY

The crowd surrounds The Biebs.

A lot has to do with what's happening in the heads of tweens and teens. In those early adolescent years from 10 to 14, kids are searching for new people to love outside of their family and they might glom on to a celebrity crush, suggests Pitts.

In addition, being around groups of young people may also make this behavior more "contagious" during a phase when peer pressure rules. For the 15- and 16-year old Bieber fans in the audience, there's also the growing interest in forming relationships and sexual attraction that fuels the emotional excitement of seeing your teen idol.

"The teen brain is still significantly changing and not yet in its final form," points out Pitts. "And that affects how teens respond to the world around them."

Young girls are especially prone to getting emotional and when they get excited, they cry "happy tears."

"I don't know that there's a scientific basis for that," admits Pitts. "It comes down to the chemistry in our brains and nerves."

As for keeling over, Pitts says fainting is really common in teens, more so in females.

If a young Bieber fan has been standing outside for hours, and they're sleep deprived and very excited, and perhaps also hungry and cold, then passing out is a possibility. It's more of an emotional faint, in which falling over is actually protective allowing blood to quickly shunt back to the brain.

Of course, this generation is not the first to weep, screech, and faint at its teenage heartthrobs. These behaviors go way back. If it wasn't the Biebs who was bringing on the waterworks and hysteria, it was the Jonas Brothers, or ‘N Sync; the Jackson 5, the Beatles or Elvis.

So why don't young boys scream and swoon at a Miley Cyrus or Selena Gomez concert? "Boy's brains and hormones are wired differently," says Pitts. And while it's culturally OK for girls to cry, that's not the behavioral norm for guys.

Then again, maybe it's just that chicks are the bigger Bieber "believers." 

More on Justin Bieber:

• Surviving a concert camp-out: 7 tips from Bieber fans
• Matt Lauer joins Twitter (with some help from Bieber)
• Video: Justin and 'Mrs. Bieber' chat with Matt Lauer
• Watch his TODAY performance

More from Body Odd:

• Yikes! Jet lag and exhaustion can make you vomit
• Elvis song may reveal clues to genetic disorder

Want more weird health news? Find The Body Odd on Facebook.

Max Rossi / Reuters

New study uncovers why fatty foods make you less sad.

A bowl of ice cream. A giant serving of mac 'n' cheese. A plate of chocolate chip cookies. We already know that fatty, comfort foods can help bust a bad mood -- but new research from Belgian researchers gives new insight into why.

Turns out, there may be more at play here than the visual, psychological effects of seeing the ice cream (or the pasta, or the cookies, or -- insert your go-to comfort food here). The study, led by Dr. Lukas Van Oudenhove of the University of Leuven and just published in the Journal of Clinical Investigation, suggests that it's the fatty acid itself in those foods that works to decrease sad emotions.

Researchers recruited 12 healthy, non-obese volunteers, who were studied the morning after a 12-hour fast. Each individual underwent four 40-minute fMRI scans (that's functional magnetic resonance imaging -- or scans that measure brain activity). Here's where it gets kind of lab-rat-like: The participants were hooked up to a gastric feeding tube that administered 250 milliliters of either fatty acid or a saline solution. But three minutes before they were given the intragastric infusion, researchers played the volunteers 11 pieces of sad classical music, of 1-minute duration each. At the same time, 10 sad faces were projected onto a screen in front of the volunteers.

Without knowing they'd just been gotten a hit of fatty acid, right to the gut, those volunteers reported feeling 50 percent less sad than the ones who got the salt solution. The researchers also found that the infusion of the fat solution to the stomach lessened the behavioral and nerve cell responses to sad emotions. As Van Oudenhove explains, the novelty of this study is that the researchers believe it was the first to determine that fatty acid itself works to brighten a bad mood -- even without the visual cues that come from seeing a cheeseburger or a cupcake.

OK, but it's still a lot more fun to indulge in your favorite foods when you're sad. What's your favorite comfort food? Leave a comment telling us what it is and whether it cheers you up.

Follow msnbc.com health writer on Twitter: @melissadahl.

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